FRAME_ALLOCATOR global, make XunilFrameAllocator not hold Limine entries
so it can be used without lifetimes, implement the process struct, add
user heap by giving back heap_start from ELF and adding sbrk syscall,
align ELF loading in userspace_stub, implement lots of libc functions in
libxunil, remove x86_64 dependency from libxunil, add malloc and all
required heap functions to libxunil and more syscall numbers, add a util
file to libxunil, add build scripts for libxunil and doomgeneric
with_process to interact with them. User space now has it's own address
space and mapper which means we will be able to allocate memory for it.
I added a bunch of functions as stubs into libxunil which are required
for doomgeneric.
